<html>

<head>
    <meta charset="UTF-8">
    <title></title>
</head>

<body>

    <div id="countdown">还剩 30 分 00 秒</div>

    <script>
        window.onload = function() {

            var countdown = document.getElementById("countdown");

            var time = 1800; //30分钟换算成1800秒


            setInterval(function() {
                time = time - 1;
                var minute = parseInt(time / 60);
                var second = parseInt(time % 60);
                countdown.innerHTML = '还剩' + minute + '分' + second + '秒';

            }, 1000);
        }
    </script>

</body>

</html>